The ability to taste the bitter compound phenylthiocarbamide (PTC) and related chemicals is bimodal, and all human populations tested to date contain some people who can (tasters) and some people who cannot taste (nontasters) PTC, e.g., the frequency of tasters in North Americans of European ancestry is about 67%. Why this trait has been maintained in the population is uncertain but this polymorphism may have evolved as a key defense mechanism against the ingestion of harmful substances. The gene that gives rise to this phenotype is unknown, and its characterization would permit insights into the mechanism of bitter taste perception and screening for modulators of taste that would be useful in the pharmaceutical, food, and beverage industries to customize taste.